A new macroscopically degenerate ground state in the spin ice compound Dy₂Ti₂O₇ under a magnetic field

The low-temperature magnetic properties of the pyrochlore compound Dy₂Ti₂O₇ in magnetic fields applied along the direction are reported. Below 1 K, a clear plateau has been observed in the magnetization process in the field range 2-9 kOe, followed by a sharp moment jump at around 10 kOe that corresponds to a breaking of the spin ice state. We found that the plateau state is disordered, with a residual entropy of nearly half the value for the zerofield state, whose macroscopic degeneracy comes from a frustration of the spins on the Kagomé layers perpendicular to the magnetic field.